1. Technical Field of the Invention
The present invention relates generally to flow control devices and, more particularly, to a cascade trim control valve which includes a uniquely configured valve stem and corresponding valve cage formed with complimentary flow passages which are selectively brought in and out of alignment with each other to vary the fluid control properties of the control valve.
2. Description of the Related Art
There is currently known in the prior art a type of control valve commonly referred to as a pintle valve wherein the short extension of a needle-valve tip is used to facilitate the control of fluid through the valve. An exemplary pintle-type fluid flow control device or control valve is described with particularity in Applicant's U.S. Pat. No. 5,803,119 entitled FLUID FLOW CONTROL DEVICE issued Sep. 8, 1998.
In one embodiment of the control valve described in the '119 Patent, a cylindrical plug is moveable axially within a complimentary cylinder. The plug includes a fluid flow path extending axially therein. As the plug is moved axially relative to the cylinder, the area of path entry available for fluid flow and the length of the fluid flow path is varied. As a result, as the plug moves further out of the cylinder, a corresponding increase in fluid flow is obtained in the control valve.
Another species of conventional fluid control valves generally includes a cage and a valve stem reciprocally moveable within the cage. The cage is typically formed from a plurality of stacked disks, which when stacked, form a plurality of tortuous flow passages between an inner surface and an outer surface. When the valve stem is in a closed position, the stem typically covers all of the passages in the cage, thereby preventing fluid from flowing through the cage. The incremental movement of the stem from the closed position toward an open position incrementally exposes the fluid passages in the cage, thereby allowing fluid to flow therethrough.
One particular concern with such valves is cavitation, which may be triggered in response to certain flow conditions. Cavitation creates damage to the structure of the valve. Another drawback with conventional disk-stack type valves is that they tend to be ineffective when used with dirty fluids, i.e., fluids having debris/particulate in the fluid. As the fluid flows through the valve, the debris may clog the passageways within the cage, thereby altering the fluid control properties of the cage.
The flow control device or control valve constructed in accordance with the present invention represents an improvement over the control valve described in the '119 Patent, and the conventional disk-stack type valves, by virtue of its inclusion of more intricate and complex flow passages which are formed in both the valve stem and the surrounding cylinder (i.e., the cage). The increased intricacy/complexity of the flow passages within the control valve of the present invention provides for a better flow range/rangeability therethrough. These, as well as other features and advantages of the present invention, will be described in more detail below.